linux-kernel - [PATCH v2 3/3] kernel/watch_queue: Remove wqueue->defunct and use pipe for clear check

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Date:   Thu,  4 Aug 2022 20:11:52 +0530
From:   Siddh Raman Pant <code@...dh.me>
To:     code@...dh.me
Cc:     christophe.jaillet@...adoo.fr, corbet@....net, dhowells@...hat.com,
        ebiggers@...nel.org, edumazet@...gle.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-kernel@...r.kernel.org, mchehab@...nel.org,
        rdunlap@...radead.org
Subject: [PATCH v2 3/3] kernel/watch_queue: Remove wqueue->defunct and use pipe for clear check

The sole use of wqueue->defunct is for checking if the watch queue has
been cleared, but wqueue->pipe is also NULL'd while clearing.

Thus, wqueue->defunct is superfluous, as wqueue->pipe can be checked
for NULL.

Signed-off-by: Siddh Raman Pant <code@...dh.me>
---
Changes in v2:
- The sent patch had accidentally missed removing the member
  from struct, fix that.
- Use !READ_ONCE() instead of == NULL, as said by checkpatch.pl.

 include/linux/watch_queue.h |  4 +---
 kernel/watch_queue.c        | 14 +++++---------
 2 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/include/linux/watch_queue.h b/include/linux/watch_queue.h
index c99c39ec6548..88360771c097 100644
--- a/include/linux/watch_queue.h
+++ b/include/linux/watch_queue.h
@@ -55,7 +55,7 @@ struct watch_filter {
  *
  * @rcu: RCU head
  * @filter: Filter on watch_notification::info
- * @pipe: The pipe we're using as a buffer.
+ * @pipe: The pipe we're using as a buffer, NULL when queue is cleared/closed
  * @watches: Contributory watches
  * @notes: Preallocated notifications
  * @notes_bitmap: Allocation bitmap for notes
@@ -63,7 +63,6 @@ struct watch_filter {
  * @lock: Spinlock
  * @nr_notes: Number of notes
  * @nr_pages: Number of pages in notes[]
- * @defunct: True when queues closed
  */
 struct watch_queue {
 	struct rcu_head		rcu;
@@ -76,7 +75,6 @@ struct watch_queue {
 	spinlock_t		lock;
 	unsigned int		nr_notes;
 	unsigned int		nr_pages;
-	bool			defunct;
 };
 
 /**
diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index 8999c4e3076d..825943cf74b2 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -43,7 +43,7 @@ MODULE_LICENSE("GPL");
 static inline bool lock_wqueue(struct watch_queue *wqueue)
 {
 	spin_lock_bh(&wqueue->lock);
-	if (unlikely(wqueue->defunct)) {
+	if (unlikely(!READ_ONCE(wqueue->pipe))) {
 		spin_unlock_bh(&wqueue->lock);
 		return false;
 	}
@@ -99,15 +99,12 @@ static bool post_one_notification(struct watch_queue *wqueue,
 				  struct watch_notification *n)
 {
 	void *p;
-	struct pipe_inode_info *pipe = READ_ONCE(wqueue->pipe);
+	struct pipe_inode_info *pipe = wqueue->pipe;
 	struct pipe_buffer *buf;
 	struct page *page;
 	unsigned int head, tail, mask, note, offset, len;
 	bool done = false;
 
-	if (!pipe)
-		return false;
-
 	spin_lock_irq(&pipe->rd_wait.lock);
 
 	mask = pipe->ring_size - 1;
@@ -603,10 +600,9 @@ void watch_queue_clear(struct watch_queue *wqueue)
 	rcu_read_lock();
 	spin_lock_bh(&wqueue->lock);
 
-	/* Prevent new notifications from being stored. */
-	wqueue->defunct = true;
-
-	/* This pipe will get freed by caller, and we are anyways clearing. */
+	/* This pipe will get freed by the caller free_pipe_info().
+	 * Removing this reference also prevents new notifications.
+	 */
 	wqueue->pipe = NULL;
 
 	while (!hlist_empty(&wqueue->watches)) {
-- 
2.35.1